1. Field of the Invention
This invention relates to video signal processing circuits and, in particular, to such circuits for increasing the amplitude of the video signal and in particular for increasing for an initial warm-up period, the biasing voltage to a video amplifier of a TV receiver.
2. Description of the Prior Art
In TV receivers of the prior art, it has been desired to provide a TV image as rapidly as possible after the TV receiver has been turned on. With early TV receivers, the viewer experienced the frustration of turning on the receiver and waiting for a relatively long period of time until the components amplifying the video signal warmed-up to a temperature at which they would amplify sufficiently the video signal, and until the cathode ray tube (CRT) and in particular its cathode element became sufficiently heated so that it would emit the image forming electron beam. An early approach to achieve a rapid display of the TV image involved the continuous application of a current (even when the TV receiver was not otherwise in use) to the cathode heater of the CRT, whereby the cathode element was maintained in a condition to emit electrons upon the activation of the remaining parts of the television receiver. However, the drawback to such a solution was the requirement of a small transformer to apply the heating current to the cathode heater, as well as the continuous application and use of electrical energy to maintain the cathode heater heated.
As it became more desired to conserve electrical energy, the continuous energization of the cathode heater became less desirable. As a result, other methods were developed for minimizing the time required to achieve a TV display. To this end, special CRT's were developed wherein the heater mechanisms were specially designed to achieve a low thermal mass so that they could be rapidly heated to cause the cathodes emissive material to emit within a relatively short period of time. In a further approach, it was suggested to apply additional power to the cathode heater element whereby it would warm-up at an increased rate. However, both of these approaches involved a cathode ray tube that was relatively expensive and whose expected life was shorter than other conventional types television CRT's. In part the reduced life resulted from the fast warm-up of the cathode element whereby repeated thermal stress was applied to the cathode element due to uneven heating; as a result, the cathode elements of such special CRT's tended to mechanically break-down in a shorter period of time.
In addition to some compensation for the time the CRT takes to warm up and its emission to stabilize, it is important that compensation also be provided for the lower current that the video transistors draw until they have warmed up to their operating temperature. This is most important where direct coupled video stages determine the CRT bias, which in turn affects the image displayed upon the CRT.